"Rapid prototyping" is a term commonly used to describe the deposition of small amounts of a material in rows or individual droplets typically in a computer controlled pattern to fabricate an article by the layer-by-layer buildup of the deposited material. Referring to FIG. 1, there is shown in simplified schematic diagram form a typical apparatus 10 used in the fused deposition process for rapid prototyping. The fused deposition apparatus 10 is coupled to a feed mechanism (not shown for simplicity) from which a filament 12 of the material to be deposited is provided. Filament 12 is fed through a pair of oppositely rotating rollers 14a, 14b, through an insulator 16, and into a liquefier 18 which includes a heater (also not shown) for reducing the material to a viscous, flowable form and further includes a discharge nozzle 20. After the material is heated within liquefier 18, it is forced out of the liquefier by conventional means and through nozzle 20 so as to form the fused deposition material 24 which is displaced in the direction of arrow 22 after discharge from the liquefier. Control is usually provided for both the flow mechanism including liquefier 18 and the movement of nozzle 20. During the deposition process, it may occasionally be necessary to change the flow rate and/or change or replace nozzle 20 particularly when more precise control of the deposition such as around the extremities of the article is required. When a nozzle is changed, the deposition process is halted temporarily. This frequently results in voids, cold joints or other defects which substantially reduces the quality and usefulness of the resulting article. Control of the flow rate without interrupting the process would represent a substantial improvement over present approaches because rapid prototype is frequently used to fabricate products of relatively high value with complex shapes and numerous passages or other openings. In addition, items fabricated by rapid prototyping also must typically be provided with highly accurate dimensions. Examples of nozzle openings used in current rapid prototyping equipment are in the range of 0.010-0.075 inch diameters.
Another problem encountered in present rapid prototyping approaches where the viscous material flows as a non-Newtonian fluid is known as "die swell". Die swell is a phenomenon involving the expansion of a fluid emanating from a nozzle or a jet, i.e., the extrudate in an extrusion process, to a diameter significantly larger than the diameter of the orifice of the jet. This phenomenon is shown in the simplified schematic diagrams of FIGS. 2 and 3 and in the photograph of FIG. 4. FIG. 4 shows the die swell of a non-Newtonian fluid as compared with a fluid flowing in accordance with Newtonian principles as shown in FIG. 5, where the fluid does not undergo an expansion as it exits the jet. In FIG. 2, the extrudate, or extruded material, 28 exits nozzle 26 having a diameter D.sub.1 and expands to a diameter of D.sub.0, where D.sub.0 &gt;D.sub.1. The inside nozzle diameter is shown as circular dot 30 in the right hand portion of FIG. 2, while the row diameter of the extrudate 28 is shown by the elliptically shaped dot 32. FIG. 3 is a simplified schematic diagram of one-half of nozzle 26 and extrudate 28 illustrating the axisymmetric jet (nozzle) radius R.sub.i and the corresponding expanded extrudate 28 radius R.sub.o. The extrudate 28 flows in the direction of arrows 34 in FIG. 3.
At present, ceramic cores formed by rapid prototyping for investment casting for items such as turbine blades may require as long as six months to complete because of the complexity of design. The cost for producing the ceramic cores using existing prototyping equipment is also significant. Current approaches also lack the dimensional control over the part being fabricated to provide a commercially acceptable article.
The present invention addresses these and other problems encountered in prior art rapid prototyping approaches using viscous materials for fabricating articles of complex shape. The inventive apparatus and method for controlling the flow rate and shape of viscous material includes a mechanical nozzle which is: (1) particularly adapted for computer control; (2) capable of maintaining positive and repeatable position of the material being deposited; (3) capable of maintaining constant internal volume of the viscous material within the nozzle to avoid fluid pressure changes; and (4) capable of quickly changing nozzle diameter, i.e. in less than one second, so as to minimize the impact of material build times.